Steam injection has significantly increased oil production in many reservoirs but frequently the efficiency of oil recovery is low because of steam channeling. Due to density differences between steam and reservoir fluids, steam rises to the top of the nonproductive steam reservoir overriding the oil body and penetrates the formation thereby creating a nonproductive steam channel. For example, in steam drive floods the rejected steam prematurely breaks through the formation and into the producing wells in such a manner that the more productive oil-bearing parts of the reservoir are short circuited. Steam therefore tends to open up channels which did not exist before and many of these channels are nonproductive. These nonproductive channels are difficult to plug, particularly for long periods of time. Most of the commonly used polymer based plugging agents used to plug off water will degrade at steam temperatures and lose their plugging capability within a few days. The more temperature stable plugging agents usually are too viscous to be effective in plugging the usually smaller and more difficult to reach nonproductive steam channels. For these reasons, long-term and indepth plugging of nonproductive steam channels is considered to be very difficult if not impossible to achieve. Consequently, a long-term plugging agent stable at steam temperatures would be of great benefit to the oil producing industry.
Presently foaming agents are used to divert steam but because of their instability at steam temperatures they tend to break down within a few days and are no longer effective for preventing steam loss. As a result, foam-forming agents usually must be injected once or twice a week in order to plug the steam channels and divert the steam into the more productive part of the reservoir. Examples of foam-forming agents and surfactants used to divert steam are sulfonates of alpha-olefins, or blends thereof such as sodium alkene sulfonate and hydroxy sulfonate, or sodium and amino oxyethylene sulfates either with or without admixing with carboxymethyl cellulose, or aliphatic sulfonates such as sodium dodecylbenzene sulfonate. Carboxymethyl cellulose or "CMC" is thought to encapsulate the steam foam and prevents the foaming agent from being activated until the CMC thermally degrades. This method was designed to permit the foam to penetrate deeper into the formation. Unfortunately the method does not appear to be particularly successful and apparently produces no better results than the use of the foam agent without CMC. Furthermore, the blending of the foaming agent with CMC ostensibly is very costly since copious quantities of materials appear to be necessary. Unfortunately most of the foaming agents break down at steam temperature and are usually only effective for up to about 4 days thereby requiring frequent reinjection of foaming agents which is both time consuming and costly.
Lignosulfonates have been proposed as a plugging agent in U.S. Pat. No. 4,074,757 but their use, it is believed, has had very limited success and relatively very large amounts are believed to be required. Although they form a permanent gel at high temperatures, in order to prevent serious damage to oil-bearing zones it is believed that it would be necessary to isolate the treatment to the nonproductive steam channels. Such isolation obviously is very difficult to achieve. These problems are perhaps part of the reason why lignosulfonates do not seem to be widely used.
Thus there is a need in the oil producing industry for a plugging agent which is stable at steam temperature, which is mobile enough to be carried deeply into the nonproductive steam channels, and which is sufficiently compatible with the oil-bearing part of the reservoir that it will not seriously damage it.
A method of reducing the flow of fluid, specifically water, has been described in U.S. Pat. No. 3,762,476 wherein a first aqueous polymer solution selected from the group consisting of polyacrylamide, a partially hydrolyzed polyacrylamide, a polysaccharide, a carboxymethylcellulose, a polyvinyl alcohol, and polystyrene sulfonate, is injected into a subterranean formation. Thereafter, a complexing ionic solution of multivalent cations and retarding anions, and which also comprises aluminum citrate, is injected into the subterranean formation. The multivalent cations are selected from the group consisting of Fe(II), Fe(III), Al(III), Ti(IV), Zn(II), Sn(IV), Ca(II), Mg(II), Cr(III), and the retarding anions are selected from the group consisting of acetate, nitrilotriacetate, tartrate, citrate, phosphate. Brine is then injected followed by a second slug of an aqueous polymer solution which can be the same or different from the first aqueous polymer solution. In any event, the complexing ionic solution of multivalent cations and retarding anions is capable of gelling both the first and second aqueous polymer solution.
U.S. Pat. No. 4,098,337 discloses a method for forming a hydroxymethylated polyacrylamide gel, in situ, to reduce the permeability of a thusly treated zone where the waterflood method of oil recovery is employed. In this case the gel was formed in situ by the injection of an aqueous polyacrylamide solution and an aqueous formaldehyde solution.
Although polyacrylamide-based gels can be effective in retarding water production or flow in some subterranean formations, polyacrylamide-based gels will not be stable or effective in all formations. In general, polyacrylamide-based gels will work satisfactorily in formations having a temperature below about 65.degree. C. Above about 65.degree. C., polyacrylamide-based gels become very sensitive to hardness of the brines, especially where hardness is above about 1000 ppm. The hardness of the water becomes a more detrimental factor the higher the temperature, thus for very hot regions even low hardness levels can render many gels ineffective. Formations which have a higher temperature, hardness, or total dissolved solids content above the aforementioned ranges usually are not capable of being successfully treated with polyacrylamide-based polymers to retard the flow of water. Thus polyacrylamide-based gels are not considered useful in preventing steam channeling.
In other flooding operations, rather than water, other fluids can be used. Some fluids which are used are carbon dioxide and steam. Because of the high temperature required in steam flooding or other steam stimulation methods, most of the gels used for blocking water are not suitable or satisfactory for blocking steam. Other steam treating methods such as "Push and Pull" operations, sometimes referred to as "cyclic steam injection" or "Huff and Puff" operations, where a production well is steamed for several days and then produced for a month or so result in steam channels being formed which if not blocked will result in an inefficient steaming operation due to loss of steam into channels which drain into nonproductive parts of the reservoir. Because many of the existing gels degrade rapidly at steam temperatures, polymers such as polyacrylamides are generally considered unsatisfactory.
Flooding operations using steam and other gases as the drive fluid frequently experience a loss of drive fluid to nonproductive parts of the reservoir because of greater ability of gases to dissipate into such channel as opposed to liquids. Loss of drive gases in flooding operations and steam in stimulation methods is more difficult to prevent because the flow channels responsible for such losses can be very small in diameter or width thereby making it very difficult to fill such channels with a blocking agent. Some viscous plugging substances, even though they may have the desired stability at higher temperatures, are not able to penetrate and effectively fill narrow channels, particularly as such channels become more distant from the wellbore.
Polyvinyl alcohol gels have been used to protect well casings from corrosion. U.S. Pat. No. 2,832,414 discloses such a method wherein an aqueous solution of a water soluble polyvinyl alcohol which is capable of forming a gel if maintained in a quiescent state, is pumped into the annular space between the casing and the wall of the bore hole. After allowing the polymer to remain quiescent over a period of time a gel is formed. The thusly formed gel prevents the intrusion of formation water into the annular space thereby reducing corrosion of the metal casing. Apparently, no crosslinking agent is employed and for that reason is not believed that this particular gel would be useful for plugging steam channels. Furthermore, in Patent No. 2,832,414 the gel is used to fill a relatively large but stagnant cavity compared to the substantial flows occurring in steam channels.
Studies of the macroscopic changes in polyvinyl acetate gels that occur upon removal from swelling equilibrium with isopropyl alcohol were reported in the Journal of Colloid and Interface Science, Vol. 90, No. 1, November 1982, pages 34 to 43. These studies were conducted using films of gels having various degrees of crosslinking and polymer concentration. The polyvinyl acetate gels were formed from precursor polyvinyl alcohol gels that were crosslinked with glutaric dialdehyde which were then converted to acetate gels by polymer homologous acetylation.
U.S. Pat. No. 3,265,657 discloses a process for preparing an aqueous polyvinyl alcohol composition, which remains fluid for at least a few seconds after preparation and spontaneously gels thereafter. The gel is formed by contacting a gelable fluid aqueous polyvinyl alcohol solution with a hexavalent chromium compound and a reductive agent to convert Cr(VI) to Cr(III). The compositions are used to produce foams suitable as insulating, acoustical, and packaging materials. The gels are crosslinked with chromium, not an aldehyde.
U.S. Pat. No. 3,658,745 discloses a hydrogel which is capable of significant expansion upon cooling in water and reversible shrinking upon heating which comprises a crosslinked acetalated hydrogel formed by reacting a polyvinyl alcohol previously dissolved in water and a monaldehyde and a dialdehyde. The hydrogels are alleged to have sufficient crosslinking to prevent imbibition of macromolecular materials such as proteins but not the imbibition of micromolecular materials such as low molecular weight water solutes. These hydrogels are alleged to be useful for dialytic purification when pure water is added to the macromolecular solution after each cycle. Apparently these particular hydrogels are able to absorb and desorb water and microsolutes with alternate cooling and heating cycles. Apparently a major amount of shrinkage of these gels occurs upon slight heating such as from 12.degree. to 37.degree. C. which indicates that these gels would have little value for preventing steam channeling in subterranean formations, especially at temperatures of 37.degree. C. or higher.